2 2x8's or 2x10 with carpet or rubber covering should be fine. the wider apart the better as this distributes weight better over the beam. 2 point loads rather than one in the center. Make sure you have keel clearance and prop shaft clearance. On your trailer measure out with a level in the areas where the keel will be over the beam to determine absolute minimum spacing then move in 2 inches. measure where your rear beam will be on the boat then measure how far down the prop shaft is. You may need 2x 12 if it is far back. Its pretty much common sense. I would expect the beams to be no more than 10 ft apart